Small electronic devices, such as those employing integrated circuits, continue to play a major role in all aspects of society. Batteries have traditionally been used to power such devices. Non-traditional energy sources include, e.g., solar power.
A disadvantage of batteries is, of course, that their energy dissipates over time, requiring battery replacement or recharging. A disadvantage of solar power is that the sun or other light sources are not always available, and the power output is generally not large.
One way to power such devices may be to use thermoelectric effects. Thermoelectric devices are based on two transport phenomena: the Seebeck effect for power generation and the Peltier effect for electronic refrigeration. If a steady temperature gradient is applied along a conducting sample, the initially uniform charge carriers' distribution is disturbed as the free carriers located at the high temperature end diffuse to the low temperature end. This results in the generation of a back emf which opposes any further diffusion current. The open circuit voltage when no current flows is the Seebeck voltage. When the junctions of a circuit formed from two dissimilar conductors (n- and p-type semiconductors) connected electrically in series but thermally in parallel are maintained at different temperatures T.sub.1 and T.sub.2, the open circuit voltage V developed is given by V=S.sub.pn (T.sub.1-T.sub.2), where S.sub.pn is the Seebeck coefficient expressed in .mu.V.K.sup.-1. The complementary Peltier effects arise when an electrical current I passes through the junction. A temperature gradient is then established across the junctions and the corresponding rate of reversible heat absorption Q is given by Q=I.sub.pn I, where II.sub.pn is the Peltier coefficient expressed in W.A.sup.- or V.
Methods employing the Seebeck effect are particularly suited to microelectronic circuits because only a small amount of power is usually necessary to power these circuits.
Previous investigators have used the Peltier effect, where an applied voltage creates a hot and cold surface, to conduct away heat in microelectronic circuits using thermoelectric ("TE") coolers. TE coolers were proposed in which an applied voltage resulted in heat conduction away from an affected zone. When a current source is used to deliver electrical power to a thermoelectric device, heat can be pumped from T.sub.1 to T.sub.2 and the device thus operates as a refrigerator. As in the case of a thermoelectric generator, the operation of a thermoelectric cooler depends solely upon the properties of the p-n thermocouple materials, expressed in terms of the figure of merit Z.sub.pn and the two temperatures T.sub.c and T.sub.h. The conversion efficiency COP of a thermoelectric refrigerator is determined by the ratio of the cooling power pumped at the cold junction to the electrical power input from the current source, and is given by: ##EQU1##
Details of such effects are provided in "Thermoelectric Power Conversion", by Jean-Pierre Fleurial; "Thermoelectric Microcoolers for Thermal Management Applications", by Fleurial, et al.; and "Films of Ni-7at. %V, Pd, Pt, and Ta-Si-N as Cu Diffusion Barriers for Bi.sub.2 Te.sub.3 ", by Kacsich, et al., all of which are incorporated by reference herein.
The thermoelectric materials are sandwiched between a pair of alumina (a well-known ceramic) substrates to form the structure. Alumina was used in this configuration to electrically isolate the thermoelectric material. These cooling devices could also utilize a diamond substrate. Diamond has the highest thermal conductivity (about 2400 Wm.sup.-1 K.sup.-1) of any known material. Diamond also has an extremely high degree of hardness, and is an excellent electrical insulator. A thermoelectric material, such as Bi.sub.2 Te.sub.3 and Bi.sub.2 Te.sub.3 -based alloys (in bulk or film form), was used as the heat-pumping component.
Alumina, Bi.sub.2 Te.sub.3 -based materials, and diamond may also be employed in the devices of the invention. In these devices, a "natural" temperature gradient may be used to generate power through the Seebeck effect rather than having an applied electric power result in heat pumped through a temperature differential. The power output of the TE device may then be used to power electronic circuits. E.g., a watch may be powered using the pre-existing temperature differential between a wearer's wrist and the ambient atmosphere. Of course, this power-generation function may be very widely applied to other devices in which a temperature differential is available.
The thermoelectric material may be Bi.sub.2 Te.sub.3 -based, and may be disposed between a first and second substrate. This material is patterned as a series of alternating n and p-type regions, or "legs", on the substrates. The legs may have square or rectangular cross sections or other suitable shape and may be arranged in a two-dimensional "checkerboard" pattern so that they are electrically in series and thermally in parallel. Of course, the voltage potential can be controlled by connecting some of the legs in parallel as well.
These devices may be constructed using microfabrication techniques. Microfabrication techniques are suitable because many hundreds or thousands of legs can be deposited and offer the potential to achieve an appropriate voltage/current combination for a given temperature differential across a device. Such techniques are also necessary to achieve the small size required for use in consumer electronics.
Films of Bi.sub.2 Te.sub.3 -based material deposited as legs on the substrates typically have thicknesses of between 1-100 microns. As may be seen in FIG. 1, the power output of the device increases (for a given cross-sectional area) as the thermoelectric legs become thinner for a given temperature difference. This results in a higher output power density. Thin films of the thermoelectric material are therefore better if the thermal resistances or electrical contact resistances are low or negligible. Thin films also allow fabrication by IC fabrication technology as mentioned above.
Electrically insulating materials having high-thermal conductivities, such as silicon carbide, aluminum nitride, boron nitride, or beryllium oxide, may be used in place of the diamond substrates. Other materials with similar electrically insulating and thermally conducting properties (i.e., as close to diamond as possible) could also be used.
The desirable properties of diamond and materials having similar properties enhance the effectiveness of the device. During operation, input heat from the hot side is rapidly and evenly spread out so that the substrate efficiently supplies heat to all the n and p legs.
The thermoelectric material employs a thermal gradient to generate power. The thermal gradient for a chronometer, e.g., a wristwatch, may be created by one side of the watch being exposed to the air (the cool side) and the other side being exposed to the wearer's wrist (the hot side) as mentioned above.
The thermoelectric material may be placed in thermal contact with the first substrate. Here, "thermal contact" and "thermally attached" can encompass any connection where heat easily flows from one material to another. This does not necessarily require that the materials be in direct contact. A metallization layer (described below) may be disposed between the substrate and the thermoelectric material to ensure that these materials are in thermal contact and mechanically attached.
A multi-layer upper stack structure can be used to attach the thermoelectric material of the first substrate. The stack structure preferably contains electrically and thermally conductive materials. Electrically conductive materials are required as they provide an in-series electrical connection between the p and n-doped legs of the thermoelectric material. A low electrical contact resistance between the electrically conductive materials and the thermoelectric legs is desirable. This reduces the total internal electrical resistance of the device and prevents degradation of its performance.
Thermally conductive materials within the stack structure facilitate heat flow between the thermoelectric material and the substrate. A low thermal resistance between the heat-dissipating device and the thermoelectric material reduces heat losses. These combined factors prevent a degradation in the device's performance. A lower stack structure having a similar multi-layer configuration (and similar electrical and thermal properties) connects the thermoelectric material to the second heat-conducting substrate.
A preferred multi-layer upper stack structure includes a metallization layer coated on the inner surface of the substrate. This thin metal coating facilitates adhesion of the substrate to other materials. In preferred embodiments, metals such as titanium or chromium are used as the substrate metallization layers. An outer diffusion barrier layer, preferably composed of ternary alloys of metal-Si-N, where the metal is a transition metal such as Ti or Ta, is then deposited on the metallization layer. The outer diffusion barrier layer prevents the diffusion of copper to the metallization layer and to the substrate. Depending on temperature, the outer diffusion barrier may not be required. E.g., at room temperature, the outer diffusion barrier may not be needed to prevent interdiffusion. A copper layer is deposited on the outer diffusion barrier layer. An inner diffusion barrier layer, preferably composed of Pt or metal-Si-N, is then deposited on the copper layer. The inner diffusion barrier layer impedes the diffusion of copper (which has a high solid-state solubility and thus diffuses rapidly) into either the metallization layers or the thermoelectric material. Impeding the diffusion of copper prevents contamination of the other materials in the stack structure. An electrical contact layer, preferably including one of the transition metals, may be deposited if required on the inner diffusion barrier layer to complete the multi-layer upper stack structure.
P and n-doped thermoelectric legs of the desired thickness are then deposited on the electrical contact layer. A second electrical contact layer, followed by a second inner diffusion barrier layer, is deposited on the legs.
Each layer of the stack structures is preferably deposited using semiconductor device fabrication techniques. Photolithography and spatially filtering masks are preferably used to pattern the layers.
The major factors which limit the power output of the device include: 1) the temperature differential across the TE legs, which is a function of the series electrical resistance of the TE legs; 2) the electrical contact resistance provided by the upper and lower multi-layer stack structures; 3) the geometry and number of legs; and 4) the thermal resistances for heat transfer at the hot and cold surfaces of the legs. An increase in the available temperature differential will increase the thermal to electrical energy conversion efficiency.
The area of the substrates are larger than the area of the p and n leg region.
Preferably, the device is in direct contact with the hot region. Alternatively, the device may be in contact with a thermally conducting material which, in turn, is in direct contact with the hot region. Heat rejection from the cold side of the TEG could be to the ambient air or to another suitably cooler medium.
Other features of the invention will be evident from the following detailed description, and from the claims.